In a turbojet, stages of vanes are mounted between stages of compressor or turbine wheels in order to straighten out the flow of the stream. These vanes are carried by the stator and they are adjustable in pitch position about their respective axes in order to optimize the flow of gas through the nozzles they constitute.
Each stator vane, or variable-pitch vane, includes a cylindrical pin for guiding it in pivoting, the pin being mounted in a cylindrical passage in the casing of the turbojet and being terminated by a drive square having engaged thereon a complementary orifice formed at one end of a rod. The other end of the rod carries a radial cylindrical finger for mounting in a control ring which surrounds the outside of the casing and which is connected to means for turning it about the axis of the turbojet, said drive means being generally constituted by an actuator or an electric motor.
The turning movement of the control ring is transmitted by the rods to the cylindrical pins of the vanes and causes them to pivot about their axes.
A certain amount of precision is required in assembling the rod with the control ring and with the vane pins in order to ensure that all of the vanes are oriented in the same manner in all of their angular positions.
In certain turbomachines, the axes of the vane pins and the axes of the rod fingers are parallel, thus enabling the rods to be mounted without clearance on the control ring and on the vane pins, by moving the rods in radial translation.
In other turbomachines, the axes of the rod fingers are radial, while the axes of the vane pins are inclined relative to a radial direction. During assembly, the rods are engaged on the control ring and on the vane pins by being moved in radial translation, thus enabling the finger of each rod to be mounted without clearance in the control ring, but requiring clearance to be provided at the other end of the rod in order to engage the orifice that is formed at said other end on the drive square provided at the end of the vane pin.
This clearance is needed during assembly because the drive square is inclined relative to the movement in radial translation of the rod, and after assembly this leads to clearance between the rod and the drive square in the longitudinal direction of the rod, and thus to significant lack of precision in the angular positioning of the vane about its axis.